Application Of Sacrificial Oxide In Devices
Sacrificial oxide has been used in IC fabrication as well in MEMS fabrication. This section reviews the use of sacrificial layer in fabrication of different devices.
Silicon wafer
'Boron diffusion
Polysilicon
Silicon dioxide
Silicon wafer
'Boron diffusion
Polysilicon
Back side etch pattern
Silicon dioxide
Upper silicon hole membrane
Back side etch pattern
Upper silicon hole membrane
- Silicon frame
FIGURE 5.4. (a) Starting wafer, (b) the hole patterned before doping, (c) Boron diffusion through the patterned hole, and back side etching, (d) Complete fabricated single-crystal silicon membrane [83].
5.4.1. Sacrificial Oxide for MEMS
In 1965, Nathanson etal. [2,80,81] used sacrificial layer technique to fabricate resonant gate transistors consistent with silicon integrated technology. They electroplated gold beam electrode on top of sacrificial layer. The sacrificial layer thickness determined the nominal beam-to-substrate distance. The sacrificial layer was etched at the end to release the beam electrode.
Howe and Muller (in 1983) used sacrificial oxide to fabricate cantilever beams from polycrystalline silicon (polysilicon) [82]. They etched holes in sacrificial oxide layer fabricated on silicon surface. Next, a polysilicon layer was deposited and plasma etched, leaving the desired cross-section to form a cantilever. Then the underlying sacrificial oxide was etched off in HF. Since a gradual step was required in the polysilicon cantilever beam to minimize the stress concentration, a tapered oxide window edge was desired. In order to achieve this, the sacrificial oxide was constructed with both the wet thermal oxidation, and CVD oxide, followed by a densification process. The oxide layer consisted of 10% thermal SiO2 and 90% phosphosilicate glass (about 8.75% phosphorous content). The thin rapidly etching surface layer needed for a tapered oxide window edge was created by low energy argon implant.
Kittilsland et al. [83] used sacrificial oxide to fabricate submicron particle filter in silicon. The fabrication steps are shown in figure 5.4. A sacrificial silicon dioxide was grown on (100) n-type silicon substrate. The thickness of this oxide determined the membrane separation distance. A polysilicon layer of 1.5 pm was deposited on top of this oxide. A silicon dioxide layer was grown on top of polysilicon layer to prevent damage during subsequent boron doping process [Fig 5.4(a)]. Holes in polysilicon and silicon dioxide were made using photolithography [Fig 5.4(b)]. Next, ahigh concentration of boron was diffused through the holes into silicon [Fig 5.4(c)]. The wafer was then etched from the back side to create opening from the bottom side. Boron doped layer acted as etch stop layer. The sacrificial oxide was then etched to form through hole in this structure [Fig 5.4(d)].
Boron diffusion
Hiiniiim
Boron diffusion
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